1. Field of the Invention
The present invention relates to a process for patterning layered thin films including at least one superconductor layer.
More particularly, the present invention relates to an improved patterning process for layered thin films comprising a bottom superconductor layer deposited on a substrate and another thin film deposited on the bottom superconductor layer and consisting of insulator, ordinary conductor or a second thin film of oxide superconductor having a different crystal orientation from the bottom superconductor layer.
2. Description of the Related Art
Oxide superconductors are expected to be used in a variety of applications due to their higher critical temperatures than conventional metal superconductors. In fact, Y-Ba-Cu-O oxide superconductor possesses the critical temperature above 80 K and Bi-Sr-Ca-Cu-O Tl-Ba-Ca-Cu-O oxide superconductors possesses those of above 100 K.
When the oxide superconductors are used in superconducting devices, it is indispensable to deposit a first thin film of oxide superconductor on a substrate and then to deposit at least one different thin film on the first thin film of oxide superconductor. The first thin film of oxide superconductor deposited directly or through buffer layer(s) on a substrate is called a bottom superconductor layer.
For example, in a superconducting junction so-called tunnel type Josephson Junction realized with oxide superconductor, it is requested to deposit the bottom superconductor layer, an intermediate layer of non-superconductor and a top superconductor layer on a common substrate successively in this order. Josephson element is a two-terminals element, so that a logical circuit consisting of Josephson junctions alone becomes complicated. In order to overcome this demerit of complexity, a variety of ideas of three-terminals elements are proposed.
In the superconductor transistors consisting of superconductor and semiconductor which is a typical three-terminals element, it is also required to combine a thin film of oxide superconductor with a thin film of semiconductor and hence successive deposition of thin films each consisting of different material is required. Successive deposition of a thin film of ordinary conductor such as metal onto the bottom superconductor layer is required also in the other type superconducting element consisting of superconductor and the ordinary conductor.
In these superconducting elements, a superconducting current passes through a thin film of the ordinary conductor or non-superconductor sandwiched between two adjacent layers of superconductors positioned close to each other. A distance between two adjacent superconductors is determined by the coherence length of superconductor. Since the coherence length of oxide superconductors is very short, the distance between two adjacent superconductors must be several nanometer.
In a layered thin film structure, each thin film is often requested to be patterned into a desired dimension and/or configuration according to its function and application.
Successive deposition of thin films onto the bottom superconducting layer must be carried out carefully in such a manner that the thin film of oxide superconductor is not deteriorated or badly influenced. In particular, when the oxide superconductor is exposed to air, composition and crystal structure of the oxide superconductor are easily spoiled, resulting in that the superconducting properties are extremely lowered.
For example, a thin film of metal is patterned into a shape of bonding pads each having a predetermined pattern at predetermined position on a superconducting thin film. The bonding pad functions as electrodes for the superconducting thin film. The thin film of metal is used also as a resistance in a superconducting circuit. In this case also, the thin film of metal which may have a thickness of less than 100 nm must be patterned into a predetermined shape without unnecessary resistance at an interface between the metal thin film and the superconducting thin film. Patterning of a thin film of oxide superconductor is also necessary in superconducting devices or integrated superconducting circuits having superconducting wiring lines.
When more than two thin films are deposited successively on a common substrate, it is usual practice to subject a surface of a bottom layer to cleaning operation before another layer is grown thereon, otherwise an undesirable junction due to contaminants adsorbed on the surface of the bottom layer or undesirable oxides produced thereon is formed at an interface between two layers and continuity of crystal growth is destroyed locally at the interface. Devices or integrated circuits having such undesirable junction or discontinuity do not show desired performance and sometimes do not work.
In the case of oxide superconductor, the surface condition of the bottom superconductor layer should be considered particularly and carefully, because the coherence length of oxide superconductors is very short. Therefore, the surface of bottom superconductor layer must be cleaned without spoiling its superconducting property and also must have well-ordered crystallinity or superconducting property.
In the field of semiconductor industries, surfaces of thin films are cleaned usually with ultra-pure water, by chemical washing, dry or wet etching or the like. In the case of oxide superconductors, however, these clearing technique can not be used due to high reactivity of oxide superconductors. If the surface of thin film of oxide superconductor is treated by these known techniques, undesirable reaction occur on the surface, resulting in that cleanness of the surface become worse and crystallinity and superconducting property are lost.
Under certain film forming conditions, constituent elements of the top superconductor layer diffuse or migrate into the bottom superconductor layer, resulting in that an interface between adjacent two layers become unclear and the bottom superconductor layer is deteriorated.
The patterning operation of each thin film in the layered structure must be carried out carefully. In particular, when a thin film deposited directly on the bottom superconductor layer is patterned, there is such a danger that oxide superconductor of the bottom superconductor layer is deteriorated seriously. In fact, if such thin film is patterned by lift-off technique with photo-resist and etching liquid, the bottom superconductor layer is badly influenced with them. Pattering by beam-etching technique with Cl ions beam, electron beam or the like also damage the thin film.
An object of the present invention is to solve the problems and to provide an improved process for patterning layered thin films comprising a first thin film of oxide superconductor or the bottom superconducting layer deposited on a substrate and another thin film having a predetermined pattern, both being deposited at predetermined position(s) on the substrate on this order without deteriorating the first thin film of oxide superconductor.